STI has come into widespread use in the semiconductor industry because of its space savings as compared with the old LOCOS isolation.
In an active well circuit, where the wells are actively biased to improve the device performance, a solid and constant isolation is needed between the wells. The isolation must be deeper (.about.2.mu.m) than the shallow isolation (.about.0.25 .mu.m) between devices.
It is not enough, however, to have only deep trenches because device isolation using deep trenches would require body contacts, as in silicon on insulator technology, and would consume a great deal of space.
In addition, it is not practical to have varying widths of deep trenches because of the nature of the filling mechanism. Ordinarily, a deep trench is filled by growth of a layer adhering to the walls of the trench, so that the thickness of the fill layer can be much less than the width of the trench. This mechanism does not work for wide deep trenches. When the width of the trench is greater than twice the depth, the thickness of the fill layer must be at least the depth of the trench; i.e. a wide deep trench would require a 2 .mu.m thick fill layer.
The combination of shallow and deep trenches has been suggested in the past, but the previous approaches have suffered from various practical problems.
If the deep trenches are not self-aligned to the shallow trenches, the deep trench can overlap the devices that are isolated by the trenches, so that the device size is not correct for that process.